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Although the probability of intelligent life existing on a given planet is very low, the sheer size of the universe more than makes up for it.
"Must" implies a 100% chance. Although the likelihood is very high, it can't be 100%. There is a chance, however unlikely, that we are the only life currently in existence at this moment.
There is no scientific evidence to suggest that there is intelligent extraterrestrial life currently existing in the universe.
If life requires an Earth-like planet, there are thousands upon thousands just in our galaxy
There are around 200 million stars in the Milky Way at the least. There are around 200 billion galaxies in the Universe at the least. Thus, there are 40 quintillion stars in the Universe at the least, each possibly containing life.
The apparition of life could be the great filter.
The probability of other life in the universe keeps falling. Before any exo-planets were discovered, the probability of life on a planet was 1/9 (we have not discovered life on any other planets). As we discover more exo-planets (and don't find life) the probability declines - now approx 1/3700.
In such a vast space you never know when you can bump into someone. There is an infinite probability that there coexists other races in this universe.
no primate on this space rock/et-ship can quite comprehend infinite space. Everything you can imagine is out there, just not likely to cross paths. It wasn't that long ago the churches were burning people for saying earth does not revolve around the sun. Infinite!